The present invention generally relates to image printing apparatus and methods, and more particularly to a digital ink jet printing apparatus and method for variable gray scale printing on a receiver medium in such a manner that xe2x80x9cquantization errorsxe2x80x9d, visible noise, excessive ink laydown and printing time are reduced.
An ink jet printer produces images on a receiver medium by ejecting ink droplets onto the receiver medium in an imagewise fashion. The advantages of non-impact, low-noise, low energy use, and low cost operation and the capability of the printer to print on plain paper are largely responsible for the wide acceptance of ink jet printers in the marketplace.
However, a disadvantage associated with ink jet printers is that ink is ejected in discrete ink droplet(s); therefore, ink jet printers only print discrete ink dots and at discrete optical density levels. Hence, unintended and extraneous ink dots, which are most visible in low-density areas of the image on the receiver medium, give rise to undesirable image artifacts. Furthermore, due to the optical densities being discrete, an input digital image with substantially continuous or high bit-depth pixel values are typically xe2x80x9cquantizedxe2x80x9d to the available density values in the ink jet printer apparatus. However, when pixel values are quantized, xe2x80x9cquantization errorsxe2x80x9d occur. These quantization errors often produce image artifacts that appear to the viewer of the image as posterization or contouring. Although quantization errors can be reduced by increasing the available levels of optical densities, reduction of quantization errors increase printing time and ink laydown. Increased ink laydown on the receiver medium requires higher liquid absorption by the receiver medium, which is undesirable.
Methods for increasing available levels of optical densities have been disclosed in the prior art. A method to increase available levels of optical densities is disclosed in U.S. Pat. No. 4,714,935. According to this patent, the volume of an ejected ink droplet is varied by modulating the electronic waveforms that activate ink drop formation and ejection. Individual droplets are ejected from an ink jet nozzle and land separately on a receiver medium. This patent also discloses that ink volume can be modulated by a single electric pulse, or by a plurality of electric pulses which increase the control dynamic range.
U.S. Pat. No. 4,513,299 discloses an alternative approach for increasing available levels of optical densities. According to this patent, a plurality of electric pulses are used to produce a plurality of ink droplets that are ejected from an ink nozzle and merge together into one droplet before arriving at a receiver medium.
In U.S. Pat. No. 5,610,637, more than one ink droplet is placed at a pixel location on the receiver medium so that an ink dot of variable diameter is formed on the receiver medium.
Disadvantages of the techniques disclosed in U.S. Pat. Nos. 4,714,935, 4,513,299 and 5,610,637 are the increased printing time and increased ink laydown on the receiver medium. As previously mentioned, increased ink laydown on the receiver medium requires higher liquid absorption by the receiver medium, which is undesirable.
U.S. Pat. No. 4,959,659 discloses yet another approach for increasing available levels of optical densities. According to this patent, a plurality of inks of different densities for each color are deposited on a receiver medium. Thus, an increase in printable ink densities is obtained by an increased number of available ink densities for each color. In addition, this method allows a plurality of ink droplets to be printed at each pixel location on the receiver medium thereby resulting in increased ink laydown. The disadvantages associated with this approach are the increased complexity and cost for the increased number of inks and ink cartridges in the printer. Another disadvantage of this method is that it also results in increased ink laydown.
The techniques disclosed by the art recited hereinabove print discrete ink droplets, and hence do not eliminate artifacts caused by quantization errors and extraneous visible discrete dots in the printed image. Therefore, problems associated with the art are quantization errors, increased ink laydown and excessive printing time.
Another problem associated with prior art ink jet printers is image artifacts and visible noise caused by manufacturing variability among the plurality of ink jet nozzles comprising a printhead. More specifically, variability in the print head fabrication process produces variability in the nozzles (e.g., variability in the size of nozzle orifices) comprising the printhead, which in turn causes undesirable variability in ink droplet volume for droplets ejected from the nozzles. As a result, some nozzles print higher densities than other nozzles. Such variability causes visible noise or image artifacts, such as banding or streaks, along the movement direction of the print head relative to the receiver medium. To reduce such visible image artifacts, typical prior art ink jet printers print in multiple shingled passes over the same image area so that each image location on the receiver medium is printed by different nozzles in each pass. In this case, print variability between nozzles is averaged out and the visual artifacts appear reduced. However, a disadvantage of this technique is that print time is greatly increased by a factor approximately equal to the number of passes.
Therefore, there has been a long-felt need to provide a digital ink jet printing apparatus and method for variable gray scale printing on a receiver medium in such a manner that xe2x80x9cquantization errorsxe2x80x9d, visible noise, excessive ink laydown and printing time are reduced.
The present invention resides in an ink jet printing apparatus adapted to receive an input image having a plurality of pixels, each pixel described by at least one pixel value respectively associated with an optical density. The apparatus comprises a printhead and at least one nozzle integrally attached to the printhead, the nozzle being capable of ejecting an ink droplet therefrom to define an associated one of the optical densities. A waveform generator is connected to the nozzle for generating an electronic waveform to be supplied to the nozzle, so that the nozzle ejects the ink droplet in response to the waveform supplied thereto. The waveform is defined by a plurality of pulses. A look-up table stores a plurality of waveform serial numbers assigned to respective waveforms, each waveform being defined by at least one predetermined parameter. A calibrator for converting the pixel values of the input image to waveform index number associated with the waveform serial number.
An object of the present invention is to provide an ink jet printer capable of reducing visible noise and image artifacts caused by extraneous discrete ink drops.
Another object of the present invention is to provide an ink jet printer capable of reproducing digital images of continuous tone for gray-scaling.
Still another object of the present invention is to reduce quantization errors.
A further object of the present invention is to provide an ink jet printer wherein printing time is reduced.
Yet another object of the present invention is to provide a suitable technique allowing selection of any one of a plurality of electronic waveforms for driving the ink jet printhead.
A feature of the present invention is the provision of a look-up table that includes electronic waveforms with respective waveform serial numbers and corresponding to desired printed optical densities, the electronic waveforms being capable of driving the print head.
Another feature of the present invention is the provision of a calibration unit in which each pixel of an image file is converted to waveform index numbers associated with waveform serial numbers.
An advantage of the present invention is that quantization errors are reduced.
Another advantage of the present invention is that visible noise caused by manufacturing variabilities among printhead nozzles is reduced.
A further advantage of the present invention is that excessive ink laydown is reduced.
Yet another advantage of the present invention is that printing time is reduced.
These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention.